Although the present invention may have other applications, it will be described in the context of a system for operating lights in a desired space or area in response to movement of a human body in a monitored area in or near the space.
The desirability of actuating lights only when they are needed has been recognized for some time and several systems for performing this task have been developed. In general terms, such a system usually includes a sensor capable of detecting the presence of a person in a space, a relay for operating lights illuminating the space and a circuit for operating that relay when the sensor detects the person and for deenergizing the relay when no person is present, or after the last person leaves the space.
In many applications, it is desirable to be able to install a circuit for this purpose to operate an existing lighting system. The circuit itself can be installed in an existing switch box in place of the original switch. This raises the problem of a usable source of current when the switch box is not wired with a neutral wire. That problem can be solved using the leakage current technique disclosed in U.S. Pat. No. 4,340,826, Muchnick, but the available power is quite low. The result is that, while the circuit will operate, it requires relatively expensive components as well as careful selection and design to minimize the power requirements.